NASA's
Hubble Space Telescope was responsible for many great discoveries
during the past decade. Nevertheless, there is much revolutionary
research left for the observatory to do. Predicting what Hubble
will discover in the next millennium is truly a challenge;
new results are generally unexpected and cannot be listed
in advance. Still, here are some science expectations for
the future:

EXPECTED
NEW DISCOVERIES FROM SPECIFIC OBSERVATIONS

1)
A front-seat view of the effects of a supernova explosion
In 1987 astronomers discovered the closest supernova seen
in four centuries. Follow-up observations by Hubble revealed
several mysterious rings of illuminated matter around the
supernova, still aglow after being flashed by the supernova's
brilliant light. During the next decade matter flying outward
from the supernova will ram into the central ring, a process
that has already started. Astronomers anticipate that a fantastic
celestial fireworks display will ensue, and Hubble, with its
unparalleled eye for detail, will give astronomers the best
view of this drama. For the first time astronomers will see
in detail how the blast wave from a supernova interacts with
the environment around it, and they expect to learn a great
deal about the structure of the exploding star, its evolutionary
history, and the nature of the enigmatic rings around it.
Since supernovae of this type are also the main source of
oxygen for the interstellar medium, from which later generations
of stars and planets form, it is important to understand this
interaction.

2)
Are there planets around stars in the oldest clusters? Globular
clusters are collections of hundreds of thousands of relatively
old stars that are deficient in heavier elements. No planets
have ever been detected around any star in a globular cluster.
Hubble will potentially detect as many as 50 planets (if they
exist) around globular cluster stars. Due to its uniquely
sharp view, Hubble can resolve as many as 40,000 stars in
one field in a globular cluster and follow the tiny variations
(of about a percent) in their brightness as giant planets
pass in front of stars. The detection of such planets will
be of enormous significance, since it will not only demonstrate
that planets can form even when there are fewer heavier elements
than in the solar system, but also that planets can survive
in crowded stellar environments. If planets are not detected,
this will also place meaningful constraints on planet formation.

3)
Filling in the missing links of life's origin Exploding stars
create the elements necessary for life, but before these elements
can be incorporated into newly formed planets and stars, they
must cycle through a galaxy's "ecosystem." Astronomers suspect
that many of the elements created in stellar explosions within
the disk of a galaxy first get blown into the galaxy's halo.
To date this halo gas has remained largely unobservable. The
Cosmic Origins Spectrograph, the most sensitive ultraviolet-light
spectrograph ever to be flown into space, will enable us for
the first time to systematically study this crucial stage
in the ecosystems of other galaxies. By observing quasar light
shining through the halos of galaxies, astronomers will obtain
"core samples" of the halo material and its composition, filling
in this important missing link in our understanding of life's
origin.

4)
A giant step towards understanding how comets form Comets
are small, icy objects that spend most of their lives well
beyond the orbit of Pluto. They are generally recognized only
on the rare occasions when one dives into the inner solar
system, thereby developing a spectacular tail. One of the
main regions where billions of comets reside is the Kuiper
belt  a region extending from about the orbit of Neptune
out to about 50 times the Earth's distance from the Sun. One
of the main challenges of any theory for the formation of
the solar system is to explain the formation and properties
of this large number of comets.

Hubble
observations of Kuiper belt objects in the infrared (using
the refurbished Near Infrared Camera and Multi-Object Spectrometer
and the Wide Field Camera 3) will determine the composition
of these objects. This will be a huge step forward in the
direction of determining their origin and formation process.
Given the fact that the evolution of life on Earth has been
dramatically influenced by comet impacts, an understanding
of the origin of comets is vital.

MORE
SPECULATIVE POTENTIAL NEW DISCOVERIES

1)
Cosmic enigmas An examination of the Hubble Deep Field North
 one of the two deepest images of the universe ever
taken in optical/ultraviolet/infrared light, revealed the
presence of a mysterious object with very unusual properties.
While the object is readily visible in near infrared light
(1.6 and 2.2 microns), it is totally undetected in visible
light (wavelengths shorter than 1.1 microns). One intriguing
possibility is that this galaxy (with a redshift of 12.5)
is far across the cosmos, existing when the universe was only
a few hundred million years old. Intervening galaxies may
have absorbed most of its light.

The
recharged Near Infrared Camera and Multi-Object Spectrometer
will reveal if the object is indeed point-like, as in the
case of a star, or has a "fuzzy" appearance, as one would
expect for a galaxy. But more importantly, the Wide Field
Camera 3 with its wide (and deep) field of view will be superb
in searching for other objects of this type, determining how
common they are, and whether they constitute a new class of
objects.

2)
"Far-out" giant planets? The dusty circumstellar disks observed
with the Near Infrared Camera and Multi-Object Spectrometer
and the Space Telescope Imaging Spectrograph reveal various
gaps and ring structures, which are potentially attributed
to the gravitational influence of large planets or protoplanets.
A "problem" with this interpretation is that the structures
are seen at large distances from the parent star, much farther
away than the known giant planets in our own solar system.
Interestingly, the analysis of some cometary orbits in the
solar system also suggests the potential existence of a perturbing
planet at a great distance from the Sun.

Are
there unexpected families of giant planets in the far outskirts
of planetary systems? Hubble surveys of faint moving objects
could reveal such new planets around the Sun, and Hubble's
highly sensitive infrared vision may even image such an object
directly.

EXPECTED
DISCOVERIES OF A MORE INCREMENTAL NATURE, BUT IN THE CONTEXT
OF A "BIG PICTURE"

1)
What makes the largest explosions in the universe? For decades
astronomers have detected bursts of gamma-rays coming from
the heavens, but until recently they had no idea where they
were coming from. During the past two years Hubble has played
a crucial role in pinpointing their origin in very distant
galaxies.

Gamma-ray
bursts are now recognized to be the largest explosions in
the universe since the Big Bang, but astronomers still do
not know what causes them. Hubble's superb vision will allow
astronomers to determine the precise location of the "bursts"
inside their host galaxies, and will thereby help to identify
the nature of the exploding objects. Furthermore, observations
in ultraviolet light shortly after the burst, and in the optical
many months after the burst (both are possible only with the
unique capabilities of Hubble), will complement observations
with the High Energy Transient Explorer II and the Chandra
X-ray Observatory. These observations will allow direct tests
of the physical processes occurring during these cosmic fireballs,
and will potentially help determine the rate at which stars
form during the cosmic history.

2)
What is the universe's ultimate fate? Edwin Hubble's discovery
of the universe's expansion in the 1920s redefined astronomers'
view of the cosmos. Until just a few years ago common wisdom
held that the gravitational pull of each galaxy on every other
galaxy must have been slowing down the expansion. However,
some recent observations of distant supernovae have led many
astronomers to wonder whether the universe's expansion is
in fact accelerating under the influence of a somewhat mysterious
repulsive force.

In
order to settle this question, astronomers are busy working
to discover and monitor ever more distant supernovae. Hubble
is absolutely critical to this effort because its superior
vision is crucial for distinguishing the supernova's light
from that of its surrounding galaxy. Only by measuring with
high precision the power of supernovae at distances spanning
half the universe's age will astronomers be able to tell if
the acceleration is real and thereby determine if the universe's
ultimate fate is infinite expansion towards a cold death.

3)
Mapping normal matter in the universe Large, ground-based
surveys are currently mapping the distribution of galaxies
in the local universe. But these galaxies represent only a
fraction of the "normal" matter  the kind that makes
up the Sun, the Earth, and human beings. A considerably larger
proportion lies in the vast spaces between galaxies. Much
of this matter is likely to be in the form of clouds that
never formed galaxies. With the Cosmic Origins Spectrograph
aboard Hubble, astronomers will be able to begin mapping out
these large gaseous clouds, which will hopefully lead to an
understanding of why some clouds form galaxies while others
do not.

4)
Where does the dark matter in the universe reside? By studying
how galaxies move in response to gravity, astronomers have
found that most of the matter in the universe is dark 
does not shine like stars. Yet, it is this dark matter that
holds galaxies and clusters of galaxies together. Astronomers
have been refining techniques to study dark matter by measuring
how severely its gravitational pull distorts light from distant
galaxies, acting like a lens.

With
the installation of the Advanced Camera for Surveys in 2001,
Hubble will gain an invaluable new tool for unmasking the
dark matter in the universe. This camera will surpass previous
Hubble instruments in the size of its field of view and sensitivity
and will take full advantage of Hubble's razor-sharp vision
to detect the minute, but telltale, distortions that signal
the presence of dark matter. In particular, the Advanced Camera
for Surveys will excel in mapping out the dark matter that
binds galaxies and clusters of galaxies. The luminous parts
of galaxies are just like the mini-lights on a huge holiday
tree, and the Advanced Camera for Surveys will enable astronomers
to see the tree itself.

5)
A broader view of the distant universe The most distant galaxies
in the universe are detectable only in infrared light. Near
Infrared Camera and Multi-Object Spectrometer observations
detected some of the most distant galaxies ever seen. In the
next few years astronomers hope to extend the frontiers of
the known universe both farther and wider, once the near-infrared
camera's cryocooler has been replaced and the infrared-sensitive
Wide Field Camera 3 has been installed. With its much wider
field of view, the Wide Field Camera 3 will allow for the
simultaneous study of unprecedented numbers of distant galaxies,
allowing astronomers to contrast and compare galaxies dating
from within a billion years of the Big Bang. All of this will
naturally pave the way for the Next Generation Space Telescope.